The present invention relates to a process for machining a three-dimensional piece by milling by means of an electroerosion device according to which
an electrode tool of cylindrical shape is driven in rotation, whose diameter is small relative to the dimensions of the surfaces to be eroded,
there is controlled by means of digital control unit the three-dimensional movement relative to the electrode-tool and said piece,
there are recorded by computer means the shapes to be machined in the form of superposition of virtual layers,
a trajectory of the electrode-tool is simulated such that this latter sweeps groove by groove, successively each of the virtual layers, and the corresponding control signals are transmitted to the digital control unit to carry out layerwise machining,
there is simulated by regulation means the longitudinal wear of the electrode tool along its path and corresponding compensation signals are transmitted toward the digital control unit so as to compensate
this longitudinal wear and to maintain the movement of the end of the electrode-tool parallel to said layers.
This machining process, commonly called EDM (Electric Discharge Machining) milling is for example described in EP 0 555 818, CH 689 182 and U.S. Pat. No. 5,911,888 of the applicant. The experience in the field of EDM milling has shown that it is possible to compensate continuously the wear of a tool that is cylindrical or in the form of a tube.
The fundamental hypothesis consists in considering that the volumetric wear is constant and the profile of the tube is substantially unvariable. As a result, by knowing the volume of material to be machined on the workpiece, there can be known in advance the wear of the electrode and hence that can be compensated stepwise.
By machining more and more complex pieces, it has been possible to determine the limits of validity of the initial hypothesis, which has proved very quickly to be insufficient when it is a matter of achieving planarity near or less than a hundredth of a millimeter.
In the case of complex geometries, the machining conditions of a piece by EDM milling can change dramatically and affect sufficiently the quantity of longitudinal wear of the electrode-tool such that the stack of successive layers gives rise to considerable errors of planarity.
The present invention has for its object to overcome these drawbacks and to obtain an EDM milling process permitting high precision and planarity of machining.
The process according to the invention is characterized to this effect by the fact that there are provided several values for longitudinal wear of the electrode-tool according to the mutual geometric configuration between the electrode-tool and the surrounding material of said piece, and that compensation signals corresponding to each of these values are transmitted to the digital control unit.
The use of several typical values for longitudinal or volumetric wear of the electrode-tool as a function of the type of geometric configuration encountered by the electrode-tool, permits obtaining a very flat movement of the end of the electrode-tool, and hence a high precision of machining.
Preferably, said values take account on the one hand of the presence or absence of an opening in the surrounding material located below the end of the electrode-tool, facilitating the flow of the machining liquid, and on the other hand of the presence or absence of at least one or two walls of the surrounding material located at a distance from the electrode-tool which is smaller than a first predetermined distance.
Thanks to these characteristics, it is possible to take account of the principal factors influencing the wear of the electrode-tool and hence increasing precision of the EDM milling.
According to a preferred embodiment, the values of longitudinal wear are fixed for the following geometric configurations:
a first configuration defined by the presence of at least two walls spaced apart a distance less than a second maximum distance predetermined by the absence of an opening below the electrode-tool;
a second configuration defined by the presence of at least two walls spaced by a distance less than a second predetermined maximum distance and by the presence of an opening below the electrode-tool;
a third configuration defined by the absence of a wall located at a distance less than the first predetermined distance of the electrode-tool and the presence of an opening below the electrode-tool;
a fourth configuration defined by the presence of a wall located at a distance less than the first predetermined distance of the electrode-tool and the presence of an opening below the electrode-tool;
a fifth configuration defined by the presence of a wall located at a distance less than the first predetermined distance and the absence of an opening below the electrode-tool;
a sixth configuration defined by the absence of a wall located at a distance less than the first predetermined distance and by the absence of an opening below the electrode-tool.
Preferably, the values of longitudinal wear are also fixed for the following configurations:
a seventh configuration to machine the material left at the end of the preceding geometric configurations and between these latter; and
an eighth configuration to carry out one or several finishing machinings.
This small number of cases of geometric configurations permits segmenting or deconstructing the machining into regions within which the longitudinal wear or volumetric wear can be approached with excellent precision.
Preferably, there is fixed for each of the geometric configurations a value of longitudinal wear by machining with pre-established values, by measuring the actual longitudinal wear for the geometric configurations and by replacing the pre-established values by the measured values of the longitudinal wear.
These characteristics permit correcting variations of longitudinal or volumetric wear due to numerous other factors, for example variations of temperature, of the degree of contamination of the machining liquid by machining waste, of the degradation of carbonated machining liquids due to electroerosive discharge, etc.
The present invention generally relates to a device for machining by electroerosion to machine a three-dimensional piece by milling by electroerosion by layers, comprising
a member to drive in rotation an electrode-tool of cylindrical shape whose diameter is small relative to the dimensions of the surfaces to be eroded,
a digital control unit adapted to control the three-dimensional relative movement of the electrode-tool and the piece,
a computer module permitting recording the shapes to be machined in the form of a superposition of virtual layers,
a simulation module permitting simulating a trajectory of the electrode-tool such that the latter sweeps groove by groove, successively each of the virtual layers and arranged to transmit corresponding control signals to the digital control unit,
a regulation module permitting simulating the longitudinal wear of the electrode-tool and its compensation along its path and to transmit toward the digital control unit compensation signals so as to regulate and compensate this longitudinal wear to maintain the movement of the end of the electrode-tool parallel to said layers, characterized by the fact that the regulation module is arranged to provide several values of longitudinal wear of the electrode-tool according to the mutual geometric configuration between the electrode-tool and the surrounding material of the piece and to transmit compensation signals corresponding to each of these values to the digital control unit.
Other advantages will appear from the characteristics expressed in the dependent claims and from the description given hereafter of the invention in greater detail, with reference to the drawings, which show schematically and by way of example one embodiment.